There has been a problem of cheat using a modified client software (hereinafter may be referred to as a “cheating client”) on a system that provides Internet services such as online games. For example, in a system providing an online game, there has been an issue that an user who tries to cheat on the game creates a cheating client in which data is modified such that items, characters and the like used in the game becomes more advantageous for the user. The user uses this cheating client to progress the game improperly. Such cheating users typically modify data relating to items, characters and the like used in the game through the cheating client using legitimate user IDs of the cheating users, and then play the game through a normal client software using the same user IDs. In order to prevent such cheating acts, there has been proposed a game program to detect improper data such as data including numerical values out of a regular range, data including a prohibited character string and the like by checking the game data used in the online game (see for example, Japanese Patent Application Publication No. 2014-061422).
However, according to the above mentioned method to detect improper data, it may be necessary to check the game data actually used in the online game frequently, which may increase load on execution of the game program and lead to the performance deterioration of the online game. Therefore, it is preferable that improper communication using modified software be inhibited without checking the game data itself.